1. Field of the Invention
This invention relates to a method for transferring and heating a non-regular and/or variable wall-thickness parison. More specifically the invention relates to a method which comprises heating a plastic tubular parison cut to a predetermined length in which the parison has an inner or outer section of non-regular or variable wall-thickness to a predetermined molding temperature within a heating furnace and feeding the parison to a blow molding stage in conformity with a mold flat or angular in section.
2. Description of the Prior Art
In the blow molding of containers of the like with plastics heretofore used, a number of method have been proposed. One of these methods is called a hot parison method wherein an extruding machine or an injection molding machine is used to form resins into a tubular or bubble configuration. The resin is then subjected to blow molding while the resin is in a semi-molten state. Another of these methods is called a cold parison method wherein a material once formed into a tubular or bubble configuration is then once cooled and re-heated to a semi-molten state or up to a molding temperature, after which it is subjected to blow molding.
In either of these methods, problems remain to eliminate the irregularities in the wall-thickness of the molded articles. In forming round containers, a substantially uniform wall-thickness at the periphery of the container may be obtained somehow, whereas in forming flat containers which have a sectional periphery whose ratio of the long axis thereof to the short axis thereof is comparatively large, and if a parison round in section and having a uniform wall-thickness is used, the difference of wall-thickness between the long axis portion and the short axis portion becomes great and the wall-thickness in the direction of the long axis portion becomes thin, thereby decreasing the rigidity, strength or the like of the containers. When an attempt is made to compensate for such disadvantages by increasing the wall-thickness, the thickness in the direction of the short axis portion becomes excessively large, resulting in problems occurring such as decreased transparency and an increase in the amount of resin required, leading to higher cost.
In an effort to overcome those disadvantages as noted above, an extrusion forming stage for producing parisons has been proposed, in which the parison is prepared so as to have a thick portion in the direction of the long axis portion or an elliptical parison fitted for a metal mold is prepared prior to the step of blow molding, whereby the wall-thickness of the molded articles can be made uniform. In this case, according to the former method, that is, the hot parison method, the parison extruded and formed is immediately held by the metal mold for blow molding so that the molding can be accomplished by readily adjusting the one-sided thickness of the parison or the elliptical portion to the flattened portion of the metal mold. In the case of the latter method, that is, the cold parison method, however, this method usually involves the step of first cooling the extruded and formed parison and cutting it in a predetermined length and then re-heating the same, and as a result, it is difficult to adjust the deformed and one-sided wall-thickness direction of the parison to the deformation direction of the metal mold.